This invention relates to a rotary magnetic head cylinder device, for use in a helical scanning system video tape recorder, in which vibrations of a magnetic tape generated at times when a video head on a rotary cylinder leaves from the magnetic tape can be reduced.
In the helical scanning system video tape recorder, the magnetic tape is wound around the circumference of the rotary cylinder over approximately 180 degrees so that video signals can be recorded on or reproduced from the magnetic tape by means of the rotating magnetic heads along a direction oblique to the longitudinal direction of the magnetic tape. In this case, the front end of the magnetic head protrudes radially outwardly from the outer circumferential surface of the rotary cylinder. Accordingly, when the magnetic head is in contact with a magnetic face of the mangetic tape, a portion of the magnetic tape around the contact portion with the magnetic head is forced to be separated from the outer circumferential surface of the rotary cylinder by the magnetic head, causing a significant pressure or force to be exerted by the magnetic head on the magnetic tape. Upon leaving or disengagement of the magnetic head from the magnetic tape, the pressure or force collapses abruptly and an impact generated at that time causes the magnetic tape to vibrate. Disadvantageously, vibrations thus generated lead to a jitter which occurs during reproduction.
The occurence of jitter will now be explained by referring to the drawings. FIG. 6 illustrates, in perspective view form, a rotary magnetic head cylinder device of a conventional magnetic recording and reproducing apparatus. Referring to FIG. 6, a rotary cylinder 3 is divided into an upper cylinder 20 and a lower cylinder 21, with magnetic heads 6 interposed between the upper and lower cylinders 20 and 21. A magnetic tape 11 is helically wound around the outer circumference of the cylinder 3 over about 180 degrees by means of two guide pins 12 and 13 and is advanced in the direction B along a tape guide 22 formed at the outer circumference of the lower cylinder 21. The magnetic head(s) 6 can be rotated by itself or along with the upper cylinder 2 in a direction A to contact with the tape 11 and leaves in such a manner that a magnetic head 6 starts to coe into contact with a lower edge of the magnetic tape 11. As best seeen in FIG. 9 depicting parts of the upper and lower cylinders in a sectional view, the magnetic head 6 is mounted to the upper cylinder 20 through a head plate 23 and rotates together with the upper cylinder 20. The front end of the magnetic head 6 protrudes radially outwardly beyond the outer circumferential surfaces of the upper and lower cylinders 20 and 21 and consequently the magnetic tape 11 is forced to be spaced from the outer circumferential surfaces of the cylinders 20 and 21 at and near the contact portion with the magnetic head.
In the conventional device thus constructed, when recording or reproducing video signals on or from the magnetic tape by advancing the magnetic tape in the direction B and rotating the magnetic head in the direction A, the magnetic head 6 starts contacting with the lower edge of the magnetic tape 11 and then leaves from the upper edge of the magnetic tape 11, as shown in FIG. 6. As long as the magnetic head 6 is kept in contact with the magnetic tape 11, the magnetic head exerts a pressure or force on the magnetic tape at the contact portion thereof but when the magnetic head 6 leaves or escapes from the upper edge of the magnetic tape 11, the pressure or force collapses abruptly to give an impact on the magnetic tape 11 which causes the magnetic tape 11 to vibrate at its upper edge C (FIG. 6). Vibrations thus occurring in the tape 11 lead to an undesirable reproduction picture on a screen of a television or video monitor 14 wherein, for example, a line E which should normally be a single straight line is undulated at a region D, as shown in FIG. 7. The undulation on the picture is generally called a jitter. The result of spectral analysis of the jitter is shown by a graph in FIG. 8.